Sounds to me like their bottom dollar is $300K and you would take the house at $285K. You're not really that far apart and should be able to put a deal together where everyone feels good enough to move forward.
If I were in your shoes I would put together the strongest argument possible for why the home is worth $275K. Many of the ideas in the previous posts are good ones. Comps with a realtor obviously can be made to say whatever you want -- He's justifying $315K with them and you're at $265K...
You would at least make his decision very hard at $285-$290K...any lower and he probably would list it.
I know it sounds like I'm assuming a lot in this post but I'm a third generation realtor/investor and have been in and around thousands of negotiations. Make your strongest case for $275K. Let him sit with it for a bit. Then come back and bend to $280K or $285K. Working together would be a best case scenario for you both!